Thermal inkjet hardcopy devices such as printers, graphics plotters, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Inkjet printheads are typically attached to a housing or body of a print cartridge. The inkjet printhead ink is fed from an internal ink reservoir integral to the print cartridge or from an "off-axis" ink supply which feeds ink to the print cartridge via tubes connecting the print cartridge and ink supply. A print cartridge having an "off-axis" ink supply usually also has a very small internal ink reservoir. In either case, the housing has an ink conduit for supplying ink from an the internal ink reservoir to the printhead. Ink is then fed to the various vaporization chambers either through an elongated hole formed in the center of the bottom of the substrate, "center feed", or around the outer edges of the substrate, "edge feed". In center feed the ink then flows through a central slot in the substrate into a central manifold area formed in a barrier layer between the substrate and a nozzle member, then into a plurality of ink inlet channels, and finally into the various ink vaporization chambers. In edge feed ink from the ink reservoir flows around the outer edges of the substrate into the ink inlet channels and finally into the ink vaporization chambers. Inkjet printheads are very sensitive to particulate contamination. To deal with this problem, a filter is typically disposed in the ink fluid path between the reservoir of ink and the printhead.
In either center feed or edge feed, the flow path from the ink reservoir to the printhead inherently provides restrictions on ink flow to the ink vaporization chambers. A concern with inkjet printing is the sufficiency of ink flow to the paper or other print media. Print quality is a function of ink flow through the printhead. Too little ink on the paper or other media to be printed upon produces faded and hard-to-read documents.
To increase resolution and print quality, the printhead nozzles must be placed closer together. This requires that both heater resistors and the associated vaporization chambers be placed closer together. To increase printer throughput, the width of the printing swath is increased by placing a larger number of nozzles on the printhead. Also, printer throughput is increased by firing the heater resistors at a higher frequency. An increased number of heater resistors spaced closer together and firing at a higher frequency creates a much greater concentration of heat generation. It is necessary to remove this heat from the printhead to prevent difficulty in supplying ink to each vaporization chamber quickly.
Previous printheads when operating at a high ink ejection rates have had cooling problems because the flow of ink across the back surface of the printhead is insufficient to adequately cool the printhead. When the temperature of the printhead gets too high print quality is degraded. This is because the printhead is finely tuned to operate optimally within a narrow temperature range because ink properties and the characteristics of bubble nucleation and growth are strongly dependent on temperature and the printhead does not perform well outside this temperature range.
Air and other gas bubbles and particulate matter can also cause major problems in ink delivery systems. Ink delivery systems are capable of releasing gasses and generating bubbles, thereby causing systems to get clogged and degraded by bubbles. In the design of a good ink delivery system, it is important that techniques for eliminating or reducing bubble problems be considered. Therefore, another problem that occurs during the life of the print element is air out-gassing. Air builds up between the filter and the printhead during operation of the printhead. For printers that have a high use model, it would be preferable to have a larger volume between the filter and the printhead for the storage of air. For low use rate printers, this volume would be reduced.
There is a need for high speed printing devices, such as desktop printers, large format printers, facsimile machines and copiers. In the past, printheads have not had the ability to operate at high speed ink ejection rates required for high speed printing rates due to lack of the ability to remove the large amount of heat generated.
Accordingly, there is a need for a new ink flow design for an ink delivery system operating at high speed printing rates.